Drop bar or flat bar? Aluminum for rust prevention, steel for ride quality? How burly should it be (what weights do you put on it, do you ride off-road at all, do you ride off curbs regularly, etc)? What sort of tire clearances do you want/need? What braze-ons do you need for racks/fenders?

You are right that not many used bikes accommodate discs. Depending on your location (and hence local CL bike market), though, it may be possible. If you are using a tensioner, any most any disc-compatible frame would work. Personally, though, I think that throwing a tensioner and triple on a a IGH bike sort of defeats the purpose; you almost may as well just go with a regular derailleur system and save some money at that point.

Some places to look for inexpensive frames that get the job done (btw, I am assuming you are in the USA, if not, you will have more local brands that will likely be cheaper):
On-One: shop.titusti.com is their US website. They have decent, versatile steel frames for a good price. Paint job is a bit lacking in durability, but otherwise good. My commuter is built up from their inbred frame.
Nashbar: They have their own house brand, mainly (all?) in aluminum. Not always the best-designed, but totally functional. No options for horizontal dropouts (I know you said you wanted a tensioner, but...). I have a touring bike based upon their touring frame that I have ridden over 10k miles.
Origin 8: They have several inexpensive ($2-300 range) frames that seem decent. Many LBS's should be able to order them via JBS importers. No personal experience.

There are others, but these are a few places that I have experience with or considered when looking for a ~$200 frame for my commuter. Good luck!